The present invention relates to the measurement of unknown properties of materials, such as moisture content or dielectric constant, using microwaves and in particular to the measurement of the moisture content of a material by transmitting microwave beams through such material so that a portion of the beam energy is absorbed by the moisture or other property. This percent moisture measurement is achieved with good accuracy in spite of the presence in the test material of another microwave absorbing property or component in variable amounts, by measuring attenuation using two microwave signals of different frequency and determination of the density of the test material by measuring the phase shift of the received signal produced by one of such signals when its corresponding microwave beam is transmitted through the test material. The invention also includes a microwave antenna with two microwave signal connections for the two different frequency signals properly positioned in order to efficiently transmit or receive two microwave beams of different frequency with the same antenna without interference.
The present invention is especially useful in measuring the moisture content of foundry molding sand and coal. In the previous U.S. Pat. Nos. 3,696,079, 3,818,333 and 4,475,080 of Charles W. E. Walker issued Sept. 19, 1972, June 18, 1974 and Oct. 2, 1984, respectively, and in the paper "Instrumentation for the On Stream Analysis of Ash Content and Moisture Content in Coal Cleaning Plants" by Gunter Fauth, et al, published at the annual meeting of S.M.E. and A.I.M.E. at Los Angeles, Calif. Feb. 26 to Mar. 1, 1984, microwave moisture measurement apparatus is disclosed. However, these prior apparatus do not employ two microwave beams of different frequency to determine the amount of moisture attenuation of the microwave beams by the test material, and do not measure the phase shift of one of the two attenuated received microwave signals to determine test material density for measurement of the percent of moisture content, in the manner of the present invention. In addition, they do not show a microwave antenna which is capable of transmitting or receiving two microwave beams of different frequency at the same time without interference in the manner of the invention.
The invention is directed to the accurate measurement of the moisture content of a wide range of solid and liquid materials. In the above noted patents, it is shown that the absorption of microwave energy from a microwave beam transmitted through the material is capable of providing accurate information on its moisture content. The present inventor has found, however, that in all materials there are interfering effects or other factors present in the material, in addition to the amount of water present, which affect the microwave absorption.
All substances in the dry state produce some microwave absorption. In some, this microwave absorption is constant but in many of the materials which are industrially important, it is not constant and must be measured by an independent means if accurate measurement of moisture is to be obtained. In some cases, for example, the dry attenuation of the microwave is due to the dry substance being electrically conductive. If this is true in the macroscopic sense, the substance is probably not amenable to measurement by microwaves but many substances such as foundry molding sand and most coals are not electrically conducting in the macroscopic sense, yet contain microscopic particles or aggregates of atoms which are conductive and which, as a result, attenuate a microwave signal passed through the substance. In the case of both coal and foundry sand, this observed dry attenuation is thought to be due to elemental carbon particles, possibly in the form of small graphite crystalites. Finely divided metallic particles could have the same effect. Whatever the cause, the microwave attenuation due to such electrical conductivity is not frequency dependent, and so is markedly different from the water resonant absorption. The same is true of ionic conductivity which is another form of electrical conductivity and may arise from the presence of salts or of acids or bases in the substance being measured since any of these will dissolve in any water which is present to produce ions. Ionic conductivity will not contribute to dry attenuation but will affect the microwave attenuation so as to add to the moisture sensitivity in proportion to the ionic concentration. If this electrical conductivity of the dry material is not constant or the ionic concentration varies, then an independent measurement is needed to account for it if accurate moisture measurement is to be obtained.
In the microwave method of moisture measurement, the microwaves are passed through the test material and therefore sense a certain volume of material and are absorbed in proportion to the number of water molecules in that sense volume. The measurement signal is therefore proportional to the mass of water per unit volume in the material. To express this as percent water requires that the mass of material in the volume sensed be known. This may require a measurement of both the thickness of material through which the microwaves are passed and the density of the material in that volume.
Yet another factor which affects the microwave moisture measurement readings is that some of the water present becomes bonded to the material. This bonding may be chemical, such as hydrogen bonds or may be physical, as for example Van der Waals' forces. In either case, the water molecules so bonded are not free to rotate as free molecules and so do not exhibit the resonant interaction with microwaves. Except for hydrogen bonding of water to celulose and starch molecules which produce a square law relationship between microwave attenuation and percent water, I have found that almost all substances exhibit an interaction which appears to be a surface bonding phenomenon because it is dependent on particle size and particularly on the finest particles present. Thus, in pure silica sand there is effectively no bonding to a coarse grade but over 1 percent water bonds to 32 mesh grade sand. The net effect of the bonding in most substances, other than the organic ones mentioned above, is to halve the microwave attenuation up to the saturation level at which all the available bonds are satisfied. Beyond this point the attenuation becomes normal. For accurate moisture measurement through this saturation level, it is necessary that this level be known and that it be measured if it is not constant. This generally requires a knowledge of the fines content. Thus, in foundry molding sand it is the finely powdered Bentonite clay which establishes this level.
To eliminate these disturbing factors and for accurate moisture measurement, it is essential therefore that at least three independent mesurements be made. Only in special cases, can some of these be replaced with constant subtractors or divisors or by periodic manual adjustments as, for example when lower accuracy is acceptable, or when measuring some substances such as ammonium phosphate fertilizer in which the ionic conductivity is directly proportional to the amount of water present and so may be accounted for by a constant calibrating factor. In some other cases where moisture determination is only required over a limited range of moisture which is known to be either wholly below or wholly above the level at which bonding is saturated, it may not be necessary to measure this level.
It is therefore the specific purpose of this invention to provide the additional independent measurement means, in addition to the simple microwave attenuation, which are needed as stated above to provide accurate moisture measurement.
The present inventor has determined that the effect of dry attenuation can be eliminated by making microwave attenuation measurements at two different microwave frequencies. Because this dry attenuation is not frequency sensitive, the difference between the attenuation signals at the two different microwave frequencies is independent of the dry attenuation and depends only on the water present. This dual frequency measurement also eliminates the effect of variations in ionic conductivity when this is a concern. It is perhaps worth noting that there are some special cases such as alcohol and heavy water in which dry attenuation is frequency sensitive because these substances have their own resonant interaction with microwaves within the frequency range used for moisture measurement; clearly however, for this reason, microwaves cannot in any case be used to measure moisture in such substances unless another water resonance is available which is free of this restriction.
The present inventor has also determined that the density of the material in the microwave path can be measured using the same microwave beam as is used for one of the attenuation measurements by determining the change of phase of the microwave signal as it passes through the material. Like the attenuation, the phase change is a function of both the quantity of material in the microwave path and its content, but it is a different function so that both density and percent water can be computed. In effect, attenuation is proportional to the imaginary part of the dielectric constant .epsilon..sub.2 of the material and phase change proportional to the real part .epsilon..sub.1.
The dielectric constant .epsilon. of any material is a complex quantity as expressed by the equation: EQU .epsilon.=.epsilon..sub.1 +i.epsilon..sub.2
Where i is the square root of minus one. Both .epsilon..sub.1 and .epsilon..sub.2 are functions of both density and water content so that if density is constant, either attenuation or phase change could be used to measure percent water, but because the water resonance principally affects .epsilon..sub.2 it is more sensitive to water and therefore generally preferred, particularly at low moisture levels. In the same way, at low moisture levels, .epsilon..sub.1 is more dependent on density than on water content. Nevertheless, phase change can be preferred in some cases for moisture measurement, particularly when electrical conductivity effects are strong because these do not affect .epsilon..sub.1 and so do not interfere with phase change measurement.
The thickness of material through which the microwaves are passed is often arranged to be held constant by the geometry of the sensing system but where this thickness does vary it can readily be measured by a variety of well known means such as by a linear resistive transducer or by a linear variable differential transformer.
The bonding saturation level is only required to be known where moisture measurements are required to be made through this level because it is only under those cicumstances that two different moisture sensitivity slopes have to be used and their change over point must be known. The bonding saturation level is almost wholly dependent on the fines content of the material which, in many cases, is contributed by a single component of a mixture and the quantity of that component is known or can readily be measured by a standard technique. For example, in foundry molding sand it is the Bentonite clay which contributes the fines content and controls the bonding saturation level and a standard procedure exists for its determination.
It is also a significant part of this invention that if the measurement of the microwave phase change is not needed to determine the material density because the density is constant or is otherwise known, the phase signal can be used to measure bonding because bound water contributes the same as free water to .epsilon..sub.1 but not to .epsilon..sub.2, whence the microwave phase change is a function of total water, whereas microwave attenuation is a function only of free water.
The application of these ideas, leading to accurate moisture measurement is perhaps best understood by considering one specific case which will illustrate the method and has proved to be highly successful, namely the measurement of moisture in foundry molding sand. When the dry ingredients of such molding sand are first mixed, they cause only small microwave attenuation, but on coming in contact with hot iron, changes are produced so that when the sand is returned for re-use and its moisture content measured, it is found to attenuate the microwaves quite strongly, even when bone dry. This dry sand attenuation has been found to vary, in some cases considerably, from one batch of sand to another. The dry attenuation is, however, found to be independent of microwave frequency, at least over a two to one frequency range as for example between 10.7 GHz and 5.8 GHz and 2.45 GHz. The difference in the attenuations at the two frequencies is therefore independent of the dry sand attenuation and a function only of the water present. It is a function of the water present per unit volume and to present this as percent water it is necessary to divide by the sand density. Now the purpose for which the sand is used requires that it be highly compactable when prepared for use as molding sand. To achieve this, Bentonite clay is added to the sand which has the property that it swells when brought in contact with water. The density of foundry sand is therefore not constant and density measurement is necessary for accurate moisture determination by microwaves.
The bonding saturation level of the water in foundry sand is certainly dependent on the amount of Bentonite clay which is present in the mix, but so is the performance of the sand in its molding function; it is therefore the practice in all foundries to ensure that this is maintained and fresh Bentonite clay is added to achieve this. Provided the water measurement on return sand is done after the Bentonite clay has been so added as required, the bonding saturation level will be above the level of moisture occurring in the return sand so that measurement of this return sand by microwaves will not be affected by the bonding saturation level.
Thus, means to develop two microwave attenuation signals and one phase signal are necessary for accurate moisture measurement in foundry sand.